Not Applicable
1. Field of the Invention
The present invention concerns a reusable, microwave transmissive, impact-resistant food container adapted for warming food in a microwave oven. The container is characterized by good chemical inertness to cold and hot food, when compared to plastic containers.
2. Background Information
Almost every household and workplace has a microwave oven in which to heat or cook food or liquids. In the typical microwave oven, a magnetron functions to generate microwave energy at a frequency of about 2.5 GHz. This energy is conveyed by a wave-guide to the interior of the oven to irradiate the food placed therein. Because food having some water content tends to absorb microwave energy, this gives rise to internal molecular motion, i.e., thermal energy, which heats the food. Some food products are heated more rapidly than others in a microwave oven; but in general the cooking of food or liquids by microwave energy is much faster than by conventional heating techniques, including infrared radiation. In heating or cooking food in a microwave oven, the food is placed in a receptacle made of a microwave-safe synthetic plastic, glass or other material which is transparent or non-reactive to microwave energy; hence, it is only the food that is heated.
The conventional microwave food heating containers are generally made of a microwave-safe plastic or glass or other microwave transparent material, but these containers have certain inherent disadvantages which have not hitherto been overcome.
Glass containers are highly favored for their transparency and wear resistance. Modern glass containers can be taken from a refrigerator and placed directly into a microwave oven for heating of food. Glass containers are scour resistant, easy to clean, and dishwasher safe. However, they have certain disadvantages. They are easy to break or chip. They are good conductors of heat, so food warmed in a glass container tends to cool rapidly after removal from the microwave oven. Further, because it is difficult to visually determine the temperature of a glass container, the accidental picking up a hot glass container can result in skin bums.
Plastic containers, on the other hand, are much more resistant to breakage than glass. However, they do not have the heft or brilliance of glass, and thus are considered to be less desirable food containers. Further, they tend to impart a xe2x80x9cplasticxe2x80x9d odor or taste to food items stored in plastic containers for long periods of time. Further, when heating food such as spaghetti sauce, the sauce tends to impregnate the plastic along the top of the spaghetti sauce where the sauce gets hottest during heating. Such rings can never be removed from the plastic and create such an unattractive appearance that the damaged cookware is usually discarded, even though functional. Further yet, plastic dishes are easily scratched or gouged if cleaned with scouring powders or cleaning utensils.
In U.S. Pat. No. 5,986,248, Matsuno et al describe a food container that is generally microwave-transparent, but that comprises one or more non-transmissive insular areas at selected locations of the container so as to improve the uniformity of microwave heating of the contained food. The non-transmissive islands are preferably formed from pieces of aluminum foil or use very thin vapor deposited aluminum films on plastic foil substrates.
It has been known from the bottle art that the advantages of; glass and plastic can be combined by making a single container having two layers, an outer layer of plastic and an inner layer of glass.
For example, U.S. Pat. No. 3,767,496 (Amberg et al) teaches putting a plastic skin on a glass bottle by shrinking a sheet of heat-shrink plastic onto the bottle. Information can be printed on the plastic outer layer, and the plastic helps protect the bottle to some limited degree from chipping. If the contents of the bottle are to be heated, they are removed and poured into a pot or other heating utensil.
U.S. Pat. No. 3,903,339 (Brockway et al) teaches a glass container coated with a plastic containment film for improving the mechanical service strength and shatter resistance of the glass container. The plastic forms a thin skin, only about 2 to 5 thousandths of an inch thick, that commonly escapes notice on a casual inspection of the product. This film protects the glass from scratches which weaken the glass.
U.S. Pat. No. 4,238,041 (Jonsson et al) teaches a plastic coated glass container, made by passing a heated bottle through a fluidized bed of plastic powder. The plastic melts and adheres to the bottle, providing a scratch resistant surface.
As is apparent from the above, bottles are small-mouthed containers traditionally associated with liquids, and particularly cool liquids. Bottles are not suitable for heating, because the small mouth of a bottle provides little room for escape of steam. If one were to attempt to boil liquids in a bottle, the liquids would be violently expelled from the bottle by the rapidly expanding steam bubbles. Further, bottles can not serve as containers for foods, because it would be difficult to remove solid food through the neck of a bottle. Thus, the application of thin films or coating layers of plastic onto bottles to improve impact resistance has had no relevance to the food container art.
U.S. Pat. No. 4,315,573 (Bradley et al) teaches a method of strengthening a single use, disposable glass shipping and delivery container such as a jar for containing foodstuffs. A thin sleeve of plastic, such as a foamed thermoplastic, is applied to the heel of the jar. This layer is indicated as improving both the thermal and physical shock strength of the glass, enabling the glass container to withstand cooling and microwave heating. The container is obviously a disposable container, and is not suitable as a reusable food container.
Paliotta et al, in U.S. 4,848,541, teach a shipping and display container for use with glass, ceramic, or other frangible cookware vessels. Their container may be made from three sheets of a thermoplastic material, one of which is configured as an outer plastic container into which the cookware vessel fits loosely; a second may be an impact protection sheet extending across the mouth of the cookware vessel between the cookware vessel and a lid; and the third of which extends above the lid so as to seal both the cookware vessel and the lid within the shipping container. The three sheets may be welded around a common periphery to form a plastic container that the purchaser has to cut apart and destroy in order to get access to the cookware vessel and lid.
Various attempts have been made to improve microwave cookware. The general strategy has been to provide a thermally insulating layer of air between two containers of similar materials, which are generally either glass or plastic. For example, in U.S. Pat. No. 4,847,459 Desai teaches a bowl-shaped plastic structure for use in microwave ovens. Desai""s structure comprises two plastic shells separated by a sealed air chamber. Because the container is made of plastic, it has all the problems associated with plastic, as discussed above.
There is thus a need for a reusable food container which does not suffer any of the disadvantages of either glass or plastic, but offers the advantages of both.
It is an object of the present invention to provide a food container which can have a long useful life, which does not impart a plastic smell or taste to food contained therein even after long periods of time, which is more shatter resistant than glass, which is easily cleaned, and in which food can be heated in a microwave oven even immediately after removal from a refrigerator.
In view of the foregoing disadvantages inherent in known food containers, a food container has been developed that is surprisingly free of the above mentioned disadvantages, and that offers a surprising number of advantages. Such a food container has significant and unexpected advantages if made of an inner layer, or vessel, of glass and an outer layer, or vessel, of plastic, with the plastic layer having a thickness comparable to that of the glass and covering the bottom and at least most of the sides of the glass vessel.
Because of the presence of the external plastic layer, the container has a significantly higher thermal insulating capability than a comparable glass container. This reduces the cooling rate of cooked food and provides an extra margin of handling safety in that there is less risk of a user being burned by picking up a glass container full of hot food.
While retaining many of the advantages of a plastic container, the container has an internal layer which is inert to food. Thus, food stored in the container will not acquire the taste or smell of plastic, even after long periods of storage. Further, food heated in such a container, even if brought to a boil, will not cause blistering or discoloration of the inside surface of the container.
The food container of the present invention has a receptacle part and, preferably, a lid part which form-fittingly covers a mouth of the receptacle. The provision of the lid allows the food container of the invention to be used for food storage as well as for microwave cooking.
The receptacle part comprises an inner glass vessel or layer having an outer plastic vessel or layer in contact therewith. In some embodiments of the invention the two layers are intimately bonded together. In other embodiments, the glass and the plastic vessels fit together in a manner that allows them to be easily separated for cleaning. Both materials are selected to be microwave safe. The plastic material may have a high hardness or may be relatively soft and flexible at room temperature. Both layers may have the same transparency or color and opacity, or the layers may differ from each other, or may have designs imprinted thereon or impregnated therein.
Conventional glass bowls (e.g., made of Pyrex(copyright) #7200 borosilicate glass) are relatively heavy and have walls commonly four to five millimeters thick. Conventional plastic bowls (e.g., Rubbermaid (copyright) Model 3053) have walls about one to three millimeters thick. This range of thickness of the plastic vessel or layer is sufficient to provide resistance to impact breakage or chipping, to provide thermal protection to the user, and to prevent overly rapid cooling of cooked food. Thus, the wall thickness of the inner glass bowl can be reduced and may, in some cases, be as little as one and one half millimeters.
The glass inner layer need not be designed to be a primary stress bearing member which is merely coated with plastic in order to improve the impact resistance of the glass. Rather, the plastic layer contributes significantly to the structural integrity of the food container, thus making it possible to reduce the thickness and thus the cost of the glass vessel component. Because the food container according to the present invention is only subject to normal household stresses, and not the stresses of shipping and merchandising, it need not be as break-resistant as certain large capacity disposable food containers. This, taken with the impact resistance of plastic, indicates that the food container of the present invention can be made very light weight, yet can have a long useful life. It may be noted that although thinner glass vessels are preferred in some instances, the invention is entirely operable in cases in which the glass vessel has a conventional wall thickness of four to five millimeters.
The relative height of the glass and plastic vessels can be adjusted to provide different benefits. In some embodiments of the invention the plastic layer covers the outside of the glass layer up to the mouth or rim area and provides a sealing lip onto which a lid may be pressed when food is stored in the container. In other embodiments, such as in a drinking vessel, the plastic may also cover the mouth or rim so as to protect the user""s lips from being burned. In yet other embodiments the plastic exterior may be shorter than the glass interior so that the food does not contact the plastic or glass-plastic interface during cooking. In some such embodiments a lid may be supplied that seals onto a glass lip.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other food containers for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.